We can bring together everything that has been said
in this chapter about the evolution of life, and some of the earlier
remarks about the formation of the planet, into a fourteen-point
scenario, a script for one element of the cosmic drama as seen from
Although the scenario format enables us to describe
events in simple declarative sentences without a constant repetition
of "probably" and "most likely," bear in mind
that this is at best a theory, a consistent set of hypotheses to
account for what happened. Some of the statements are on very firm
ground, but others are outlines for future research, to which chemists
will make a major contribution.
1. The universe began roughly 20 billion years ago, and our galaxy,
the Milky Way, approximately 13 billion to 15 billion years ago.
Our sun is a second-generation star in this galaxy, formed from
the heavy-element-rich debris from earlier stars. As the new star
formed by gradual accretion of matter from a dust cloud, local nodes
of material in the plane of rotation of the flattened dust cloud
began to build up into protoplanets, moving around the sun. One
of these aggregates became our Earth.
2. The young Earth was too small to retain whatever original atmosphere
it may have had, and what remained was an airless ball of rock,
made up mainly of elements, such as silicon, oxygen, and metals,
that could form nonvolatile compounds. Heat from compression and
from natural radioactive decay caused the interior of the planet
to become fluid, leading to the stratification by density that exists
today: iron-nickel core, olivine mantle, and a crust of lighter
silicates and other minerals.